1. Field of the Invention
The present invention relates to a liquid crystal panel driving technology for a liquid crystal display, and more particularly, to a boost converter of a liquid crystal display which reduces electromagnetic interference using a variable frequency when generating a panel driving voltage, and performs a boosting operation in synchronization with a synchronizing signal of image data, thereby achieving a stable boosting operation without exerting an influence on images.
2. Description of the Related Art
FIG. 1 is a block diagram schematically illustrating the configuration of a conventional liquid crystal display. As shown in FIG. 1, the conventional liquid crystal display includes a liquid crystal panel 110 and an LCD driver IC (LDI) 120. On the liquid crystal panel 110, a plurality of gate lines and data lines are arranged perpendicular to each other to define pixel areas in the form of a matrix. The LDI 120 includes a driving circuit unit 121 for supplying driving signals and data signals to the liquid crystal panel 110, and a power supply unit 122 for supplying various power supply voltages required for the driving circuit unit 121.
The driving circuit unit 121 includes a gate driver 121A, a source driver 121B, and a timing controller 121C.
The gate driver 121A outputs a gate driving signal for driving each gate line of the liquid crystal panel 110.
The source driver 121B outputs a data signal to each data line of the liquid crystal panel 110.
The timing controller 121C controls the driving of the gate driver 121A and source driver 121B, while controlling the driving of the power supply unit 122.
The power supply unit 122 includes a power control section 122A, a source-power driving section 122B, and a gate-power driving section 122C.
The power control section 122A controls the driving of the source-power driving section 122B and gate-power driving section 122C under the control of the timing controller 121C.
In this case, the gate-power driving section 122C supplies a gate high voltage VGH and a gate low voltage VGL which are required for the gate driver 121A to generate the gate driving signal.
In addition, the source-power driving section 122B supplies a positive-polarity panel driving voltage VDDP (hereinafter, referred to as a “panel driving voltage VDDP”) and a negative-polarity panel driving voltage VDDN, which are required for the source driver 121B to generate the data signal.
FIG. 2 illustrates a boost converter circuit which is included in the source-power driving section 122B of the LDI 120, and outputs a panel driving voltage VDDP. As shown in FIG. 2, the boost converter circuit includes: a field-effect transistor FET1 for driving a reactor L1 in response to a switching pulse LSW; a condenser Cout for storing a voltage which is loaded through a blocking diode D1 from the reactor L1 according to the switching operation of the field-effect transistor FET1; resistors R1 and R2 for dividing a panel driving voltage VDDP, which is output after being stored in the condenser Cout, at a predetermined ratio; and a controller 200 for monitoring the panel driving voltage VDDP through a voltage divided by the resistors R1 and R2 and controlling the switching pulse LSW so as to output the panel driving voltage at a target level.
The field-effect transistor FET1 repeats a series of on/off operations in response to a switching pulse LSW, as shown in FIG. 3(a), which is input to the controller 200. In this case, a boosting voltage loaded from the reactor L1 by a switching operation of the field-effect transistor FET1 is stored in the condenser Cout through the blocking diode D1.
The boosting voltage stored in the condenser Cout through a path as described above is output as the panel driving voltage VDDP.
However, when the amount of current loaded to the outside through the output terminal for the panel driving voltage VDDP is less than that accumulated in the condenser Cout, the panel driving voltage VDDP increases to an unnecessarily high level.
In order to prevent such a matter, the panel driving voltage VDDP output from the condenser Cout is divided into voltages having predetermined levels through the use of resistors R1 and R2, and the controller 200 controls the switching pulse LSW so that a voltage having a target level can be output while monitoring the panel driving voltage VDDP based on the divided voltages.
The switching pulse LSW includes, for example, a pulse-width modulation (PWM) pulse and a pulse frequency modulation (PFM) pulse. When using the PWM pulse, the controller 200 controls the duty ratio of the PWM pulse so that a boosting voltage having a target level can be output. When using the PFM pulse, the controller 200 controls the frequency of the PFM pulse so that a boosting voltage having a target level can be output.
Generally, when the form of a switching pulse LSW to be output has been determined, the controller 200 outputs the switching pulse LSW with the same phase as shown in FIG. 3(a). Accordingly, a periodic boosting operation causes the spectrum to be concentrated to the band of a center frequency f0, as shown in FIG. 3(b).
As described above, according to the conventional boost converter circuit of an LDI system, a switching pulse with the same phase is used as a switching pulse for driving a reactor, and a period boosting operation causes an energy spectrum in an amplified form to be formed, so that the energy spectrum of a harmonic frequency appears in an amplified form, too.
Furthermore, the energy spectrum in an amplified form has a problem in that it causes electromagnetic interference (EMI) with the frequencies of other signals used in the system.
Recently, in order to solve such a problem, a method of generating a switching pulse with the form of a variable frequency so as to generate a spread spectrum has been proposed. However, since switching pulses with mutually different frequencies are used whenever each frame begins, panel driving voltages of different levels little by little on all such occasions, so that there is a problem in that images are unstably displayed.